I think there are a few questions you need to answer before picking the lens.
1) How many images do you want to stitch together (5, 15, or 45)?
2) How many pixels do you want to have when you are done? (put another way, how large do you want to print the image?)
3) What field of view do you want to shoot? 30, 60, 90, 180, 360 degrees?
If you want to get 360 degree panos with as few frames as possible, get a very wide lens, even a fisheye (stitching programs have no problem dealing with these images). If you want to make ultra-high resolution images that will cover an 8'x10' wall at 240 ppi, then you need a very long lens to capture all those pixels.
I shoot panos using a 17-40mm, a 50mm, and a 70-200mm lens. My largest has been 87 images stitched together, and I frequently make images with only three images stitched together.
Try doing some stitching with your current lens. You will quickly get an idea of your preferences.
Two more things to consider:
Wide angles have lots of sky and foreground that can be boring in panos.
If you see a scene you want to frame in a particular way, a prime may not give you a choice about how many images to use. Zooms are fine for panos.